How to sculpt folded skin
You can use the Texture panel in Sculpt Mode, as shown below:



This will apply a texture to your brush. If it doesn't look right, you can adjust the size of the texture under the "Size" portion of the panel. Additionally, you can use "Spacing" under the Stroke panel to adjust the sample rate(?) of the stroke.

Edit: Here is a video of the texture brush in action, at around 35 minutes in.